The present invention relates to an image enhancement system suitable for use in television (TV) receivers, video tape recorders (VTR), printers and other types of video apparatus, various types of image processing apparatus used for performing the recognition, measurement, quantification, diagnosis and other operations for images, as well as various types of data communication apparatus and the like.
Portion of an image which enhances the image is often an edge portion or a portion where the density changes, and the human eye moves towards these portions. Because of this, it is possible to improve the image quality by emphasizing these portions.
Conventionally, when image quality is improved, there is the method for improving the image quality where a secondary differential signal is made from an original signal such as a TV signal and the original and the secondary differential signals are overlapped to emphasize the edge portion.
For example, in the apparatus disclosed in U.S. Pat. No. 4,030,121 (Applied on: Dec. 2, 1975; Inventor: Yves C. Faroudja), an original signal such as a TV signal is differentiated by a first differentiator to form a first differential signal and this signal is differentiated by a secondary differentiator to form a secondary differential signal. The amplitude of this secondary differential signal is limited to a constant level by a limiter.
The first differential signal undergoes full-wave rectification by a full-wave rectifier and this full-wave rectified first differential signal and the amplitude limited secondary differential signal are multiplied together by a multiplier to form the component where the edge is emphasized.
The edge emphasized component is added to the original signal by an adder to form a TV signal where the edge is emphasized.
However, the full-wave rectified first differential signal is included as an unwanted ripple component in the edge-emphasized TV signal to distort the TV signal.
This distortion becomes a cause of deterioration of the image quality. Accordingly, with a conventional image enhancement system, if there is too much edge emphasis, the edge emphasis for improving the image quality conversely acts to deteriorate the image quality and it is extremely difficult to adjust the amount of the edge-emphasized component.
A conventional image enhancement system gives uniform image emphasis on all waveform portions of a TV signal owing to a lack of judging function of the waveform of the TV signal.
A secondary differential signal for edge emphasis, which is formed by a conventional image enhancement system, has a waveform involving peak signal portions situated further outside from mid points of edge portions of a TV signal. This causes preshoots or overshoots when the secondary differential signal is added to the TV signal. This results in a reproduced image the edge of which is hemmed in black and white.
A color runs on a reproduced image when a band width of a color signal varies, the signal being a composite video signal modulated by a color subcarrier varies. In general, a secondary differential signal is added to the color signal after demodulated into a base band signal.
As a reproduced image has been highly qualified, a video apparatus, which produces a video signal having a color signal modulated by a color subcarrier and a luminance signal, both being separated from each other, has spread. An image enhancement system which gives edge emphasis directly on the color signal modulated by the color subcarrier has been awaited.